150 MICRO-ORGANISMS IN WATER 



It will be seen that the worst result was obtained 

 with Filter II. on October 9, 1891. In this case there 

 were 6,048 in the unfiltered water, and as many as 

 5,836 in the filtrate, and it must be noted that the pres- 

 sure was as high as 650 mm., and the filter had been 

 working as long as 35 days. 



Again we find with Filter IV. an unsatisfactory re- 

 duction on August 29, 1891, The unfiltered water only 

 contained 700, but the filtrate as many as 432. Here 

 again the pressure was high, being 700 mm., and the 

 filter had been working for 26 days. Similarly with 

 Filter V. on January 5, 1891, the reduction was only 

 50 per cent., the pressure being 400 mm., and the filter 

 working for 26 days. Again, with the same filter on 

 September 25, an unsatisfactory reduction is observ- 

 able ;■ whilst in this case also the pressure was high, 

 and the filter had been working a large number of days. 

 There does not appear from the table to be any special 

 connection between the filtrate and the number of days 

 the water remained on the filters before being passed 

 through. 



The principal point which these results exhibit is 

 the great reduction in the efiiciency of filtration which 

 ensues when the pressure employed is excessive, for 

 from the results obtained in other places, notably in 

 London, there can be no doubt that the greater age of 

 the filters in which these higher pressures were used 

 was not the factor determining the inefficiency, although 

 the high pressures had, of course, to be employed in 

 consequence of the age of the filters having impaired 

 their perviousness. It is in this waj^ that in actual 

 practice the age of a filter may indirectly lead to in- 

 efficiency in filtration. 



Paisley Water Supply > — The sand filters belonging 

 to the waterworks of Paisley, N.B. have been recently 



